The present invention relates to an isotope gas measuring apparatus used in the medical field for measuring a stable isotope gas in an expiration gas. More particularly, the invention relates to an isotope gas measuring apparatus for determining concentrations of 12CO2 and 13CO2 or a ratio thereof by measuring a sample gas containing 12CO2 and 13CO2 by a non-dispersion-type infrared absorption method.
Heretofore, a method called an isotopic tracer technique has been used to study metabolism, storage, excretion and the like in vivo. Using an isotope labeled element or substance as a tracer, this technique is examining a behavior of the specific element or substance in vivo, which shows a similar behavior to that of an unlabeled element or substance. In the medical field, this technique has been used as a tool for diagnosis and research using a tracer with a stable isotope label without fear of exposure to radiation. More specifically, this technique has been used to diagnose an infection of Helicobacter Pylori (hereinafter referred to simply as xe2x80x9cPylorixe2x80x9d), which is believed to cause a gastric duodenum and/or stomach ulcer and a gastric cancer. In this method, a urea reagent containing 13C, a stable isotope element, is used and the decomposition of the urea reagent by Helicobacter Pylori is detected by measuring an expiration gas of an examinee. The xe2x80x9cPylorixe2x80x9d has a urease activity wherein urea is decomposed into carbon dioxide and ammonia. Thus, urea labeled with 13C, an isotope of 12C, is administered to an examinee, and a concentration (actually, a concentration ratio of 12CO2 and 13CO2) of 13CO2 in an expiration gas of the examinee is measured. If xe2x80x9cPylorixe2x80x9d is present in the alimentary system of the examinee, since the concentration of 13CO2 becomes higher than that of a normal person who does not have xe2x80x9cPylorixe2x80x9d, it can be determined that the examinee is infected by xe2x80x9cPylorixe2x80x9d.
Generally, as a method for measuring an isotope gas (for example, 12CO2 and 13CO2) in a gas sample, there have been known methods using a mass spectrograph and a non-dispersion infrared spectroscopy. In a method using a mass spectrograph, although an accurate measurement can be carried out, an apparatus tends to be very expensive and requires a skilled operator. On the contrary, in a method using a non-dispersion infrared spectroscopy, an apparatus is generally inexpensive due to a simpler structure, and its operation is relatively easy. Thus, it is assumed that the method using a non-dispersion infrared spectroscopy will prevail in the future.
There are three typical methods to measure a ratio of the isotope gases 12CO2 and 13CO2 using a non-dispersion infrared spectroscopy.
(1) The first method uses four gas cells, i.e. a 12CO2 measuring sample cell, a 13CO2 measuring sample cell, two control cells filled with a gas containing 12CO2 and 13CO2 at a predetermined concentration for each sample cell, and two infrared detectors. In this method, a sample gas is injected into each sample cell, and absorbance of infrared light of the sample gas is compared with those of the respective control gas to determine concentrations of 12CO2 and 13CO2.
(2) The second method uses two sample cells with interference filters and two infrared detectors for the measurement of 12CO2 and 13CO2, as disclosed in Japanese Patent No. 2996611. In this method, a sample gas and a control gas are injected one by one into a 12CO2 measuring cell and a 13CO2 measuring cell to measure absorbance of the infrared light, and compare the absorbance of the sample gas and the control gas to measure concentrations of 12CO2 and 13CO2.
(3) The third method uses only two cells for a sample and a control gas and one infrared detector with a grating, as disclosed in Japanese Patent Publication (KOKOKU) No. 3-31218. In this method, the infrared light from a light source is filtered, and only the light of absorption wavelength (about 4,250 nm for 12CO2 and about 4,415 nm for 13CO2) is introduced into the sample cell and the control cell one by one, and absorbance of the sample gas and the control gas at the respective wavelengths are measured and compared to thereby determine the concentrations of 12CO2 and 13CO2.
Among the methods for measuring a ratio of 12CO2 and 13CO2, in the case of (1), the absorbance of the infrared light passing through the control cell and the sample cell is always compared, so that a stable measurement can be obtained. However, since the two independent systems for measuring 12CO2 and 13CO2 are used, they need their own infrared light sources and infrared detectors. It is known that light intensity from a light source varies with time and an output of an infrared detector with respect to the same light intensity also varies with time. The phenomena are called drift. If a variance due to the drift in two systems is not equal, even if a sample gas has the same 12CO2 to 13CO2 ratio, results of the two systems can be different. Also, since two detectors and two control cells are required, a cost becomes high.
The above-mentioned method (2) is called a single cell system, wherein although two infrared detectors are used, one single cell is used by switching a sample gas and a control gas. Therefore, another cell is not necessary, so that a cost of an apparatus is lower than that of the method (1). However, it is required to replace a gas in the cell with another gas in order to measure two types of gases, i.e. a sample gas and a control gas. The step of replacing a gas normally takes 1 to 3 minutes. If the drift occurs in a measuring system within this time period, an accuracy of the measurement is deteriorated by the drift and becomes worse than that of the method (1). Also, as a sample gas and a control gas are measured one by one, it takes nearly two times longer than that of the method (1).
Although the method (3) comprises only a sample cell, a control cell and an infrared detector, an optical system including a grating and a mirror for obtaining infrared light with a predetermined wavelength from an infrared light source has to be provided. Such an optical system tends to be a complicated one, which results in a higher cost of an apparatus.
The present invention has been made to obviate the above disadvantages, and an object of the invention is to provide an isotope gas measuring apparatus, wherein stable and repeatable measurement with respect to a gas concentration can be achieved without influence of the drift of a light source, an optical detector and the like. Moreover, the measuring apparatus has a simple structure and can be manufactured at a lower cost.
Further objects and advantages of the invention will be apparent from the following description of the invention.
In order to achieve the above objects, the present invention provides an isotope gas measuring apparatus to determine concentrations of carbon dioxide 12CO2 and stable isotope 13CO2 thereof in a sample gas by absorbance of the infrared light. The isotope gas measuring apparatus includes: two sample cells through which a sample gas to be measured passes; a control cell filled with a gas having no infrared light absorption at a wavelength used for the measurement; a light source for irradiating infrared light into the two sample cells and the control cell; an optical coupler for coupling lights passing through the two sample cells and the A comparative cell; a first interference filter disposed between the light source and the optical coupler in a light path of one of the two sample cells; a second interference filter disposed between the light source and the optical coupler in a light path of the other sample cell; and an optical detector for detecting the infrared lights from the optical coupler.
In the present invention, the isotope gas measuring apparatus uses three cells; namely a sample cell for measuring 12CO2, a sample cell for measuring 13CO2 and a control cell. The control cell can be common for 12CO2 and 13CO2. Further, the permeated light from these three cells can be coupled into one through the optical coupler to thereby reduce the number of optical detectors to one. Thus, the sample gas and the control gas can be measured substantially at the same time, which minimizes the negative influence of the drift in the light source, the optical detector and the like. Also, since the isotope gas measuring apparatus of the invention uses the same light source and detector for measuring the permeated infrared lights from 12CO2 and 13CO2 and the control gas, the drift between the gases can be removed. Further, since the isotope gas measuring apparatus of the invention uses only three cells and one detector, a structure thereof is simple, and a cost thereof can be lowered when compared with those of the above-stated three methods.
The permeated lights from the 12CO2 measuring sample cell, the 13CO2 measuring sample cell and the control cell are coupled through the optical coupler and its absorbance is measured by the detector. A signal from each of the cells can be taken by providing a separator circuit to a processing circuit that processes a signal from the detector. For this purpose, in the case that a flashing light source is used as an infrared light source, a timing of the emission is regulated; and when a continuous emission light source and a mechanical rotating sector are used, a switching timing of the light is to be controlled.